RNA POLYMERASE-II TRANSCRIPTION FACTOR-SIII .2. FUNCTIONAL-PROPERTIESAND ROLE IN RNA CHAIN ELONGATION

A novel transcription factor that stimulates synthesis of accurately i nitiated transcripts by mammalian RNA polymerase II has been identifie d and purified to apparent homogeneity from rat liver extracts (Bradsh er, J. N., Jackson, K. W., Conaway, R. C., Conaway, J. W. (1993) J. Bi ol. Chem. 268, 25587-25593). This factor, which we designate SIII, has a native molecular mass of approximately 140 kDa and is composed of t hree polypeptides of 110, 18, and 15 kDa. In this report, we demonstra te that SIII stimulates promoter-specific transcription by increasing the overall rate of RNA chain elongation by RNA polymerase II. Results of pulse-chase experiments indicate that SIII does not need to be pre sent during preinitiation complex formation or transcription initiatio n in order to stimulate promoter-specific transcription. In addition, SIII is able to stimulate the rate of RNA chain elongation by RNA poly merase II during transcription of double stranded oligo(dC)-tailed tem plates. Taken together, these findings indicate that the factor exerts its activity directly on the elongation complex.